The present invention is directed to an improved process for manufacturing embossed nonwoven fibrous products.
Embossing treatment adds aesthetic and performance attributes to many paper and fibrous products. Embossing has been practiced both in the paper product and nonwoven fibrous product fields. In the field of air laid nonwoven fibrous products, however, high speed embossing presents unique problems.
Air laid nonwoven webs can be differentiated from "paper" products because air laid nonwoven webs do not possess hydrogen bonding for needed product strength. Instead, air laid nonwoven webs are bonded by latexes, starches, or thermoplastic binders. U.S. Pat. No. 3,575,749 to Kroyer discloses methods for making fibrous sheets or webs.
The Kroyer patent teaches as part of the Background of the Invention that paper-like sheets or webs can be made by forming, on an endless metal band, a binder film and supplying to the binder film cellulosic fibers which form a uniform fiber layer on the binder film by means of an electrostatic field.
The Kroyer patent discloses another method for forming cellulosic fiber sheets or webs in which the fibers are deposited upon a forming surface which may be a foraminous metal band or other type of gas permeable band such as a porous scrim. A stream of gas containing suspended fibers is passed through the forming surface to form a fiber layer thereon. The fibers of the fiber layer are bonded together by applying a binder. The method of Kroyer thus produces a continuous sheet of fibrous material.
Air laid nonwoven webs, including those taught by Kroyer, are sometimes subjected to an embossing step to add aesthetic and performance attributes to the finished product.
The existing embossing techniques for air laid nonwoven fibrous materials fall into two general categories. The embossing step may be carried out prior to the binder application, which is commonly referred to as "pre-embossing." The second method is to carry out an embossing step after the binder material is applied, dried and set. This method is known as "post-embossing."
The "pre-embossing" technique is disclosed in U.S. Pat. No. 4,135,024 to Callahan et al. In a pre-embossing method, an air laid nonwoven web is subjected to embossing by concurrently passing it through a nip formed by an embossing roll and an anvil roll prior to applying any binder to the web. The web at this stage, not being binder treated nor cured, is weakly bonded. The weakness of the web prohibits embossing of the web at reasonable production speeds. The weak web causes special handling problems which can only be remedied by special requirements such as web carrier, web re-enforcements, or long fiber addition which cause loss of production speed and increased cost. Further, the embossed substrates suffer a permeability loss, which in turn, decreases the drying efficiency since more energy is required for drying. In the subsequent binder application, the embossed fibers tend to relax and cause a reduction in embossing definition and clarity because the relaxed fibers tend to "spring back."
The "post-embossing" technique subjects the web to an embossing step after it is treated with a bonding agent and dried and cured. The post-embossing method eliminates web handling difficulty, as well as spring-back and drying problems. Production speed can be increased because of the increased strength of the strongly bonded web. This method, however, is unsatisfactory because good embossing definition and high embossing quality cannot be achieved. The binder treated web, once dried and set, becomes resilient to pressure and deformation enabling the web to resist embossing.